1. Field of the Invention
The present invention relates to an image processing apparatus and method. In particular, the present invention relates to an image processing apparatus and method, which can more appropriately perform a local control of a filter process during an encoding or decoding operation.
2. Description of the Related Art
Recently, devices complied with an MPEG (Moving Picture Experts Group) system or the like, which manage image information as digital information, and compress, for the purpose of transmitting and accumulating information having high efficiency, the image information by an orthogonal transform such as a discrete cosine transform and motion compensation using redundancy peculiar to the image information, have been widely used on both sides of a broadcasting station for transferring the information and general home for receiving the information.
In particular, MPEG-2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) has been defined as a general purpose image encoding method, and has been widely used in widespread applications for professional use and consumer use as the standard encompassing both an interlaced scan image and a progressive scan image, a standard-resolution image, and high-definition image. By using the MPEG-2 compression method, for example, by assigning a code rate (bit rate) of 4 to 8 Mbps to a standard-resolution interlaced scan image having 720×480 pixels and a code rate of 18 to 22 Mbps to a high-resolution interlaced scan image having 1920×1038 pixels, a high compression rate and a good picture quality can be realized.
Although MPEG-2 is mainly for the high picture quality encoding that is suitable to broadcasting, it does not yet cope with the code rate (bit rate) that is lower than MPEG-1, that is, the encoding method having a higher compression rate. With the spread of portable terminals, the necessity for such an encoding method is considered to have increased hereafter, and to cope with this, the standardization of the MPEG-4 encoding method has been performed. With respect to such an image encoding method, the standard thereof has been approved as an international standard in ISO/IEC 14496-2 as of December, 1998.
Further, recently, for the original purpose of performing image encoding for a video conference, it has been known that the standard called H.26L (ITU-T (ITU Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Experts Group)) has recently been standardized. In comparison to the encoding method such as MPEG-2 or MPEG-4 in the related art, H.26L necessitates a larger amount of computation in encoding and decoding operations, but can realize higher encoding efficiency. Also, currently, as a part of MPEG-4 standardization activities, the standardization to realize higher encoding efficiency through adoption of even functions that are not supported in the H.26L, on the basis of the H.26L, has been made as the Joint Model of Enhanced-Compression Video Coding. According to the schedule of standardization, international standards based on the names of H.264 and MPEG-4 Part 10 (AVC (Advanced Video Coding)) have been made as of May, 2003.
Also, as the next-generation video encoding technology having been recently examined, there is an adaptive loop filter (ALF) (for example, see Yi-Jen Chiu and L. Xu, “Adaptive (Wiener) Filter for Video Compression,” ITU-T SG16 Contribution, C437, Geneva, April 2008). According to this adaptive loop filter, the optimum filter process is performed for each frame, and block distortion, which could not have been completely caught by a deblocking filter, or distortion caused by quantization can be reduced.
However, since it is general that an image has diverse features locally, optimum filter coefficients also differ locally. In a method described in Yi-Jen Chiu and L. Xu, “Adaptive (Wiener) Filter for Video Compression,” ITU-T SG16 Contribution, C437, Geneva, April 2008, since the same filter coefficient is applied to all pixels in a frame, the picture quality is improved with respect to the whole frame, but may deteriorate locally.
Accordingly, methods which do not perform the filter process in a region where the picture quality deteriorates locally have been considered (for example, see Takeshi. Chujoh, et al., “Block-based Adaptive Loop Filter” ITU-T SG16 Q6 VCEG Contribution, AI18, Germany, July, 2008 and T. Chujoh, N. Wada and G. Yasuda, “Quadtree-based Adaptive Loop Filter,” ITU-T SG16 Q6 VCEG Contribution, VCEG-AK22(rl), Japan, April, 2009). In these methods, the image encoding apparatus makes a plurality of control blocks, which are closely arranged over the whole surface of an image region, correspond to the image region, and controls whether or not to perform the filter process with respect to the image for each control block. The image encoding apparatus sets flag information for each block, and perform an adaptive filter process according to the flag information. In the same manner, the image decoding apparatus performs an adaptive filter process based on the flag information.